This invention relates to the coating of powders with valve-metal, to the coating of such coated powders with further valve-metal coatings, and to the production of capacitors from the coated powder so formed.
It is known for instance from British Pat. No. 1,030,004 to make capacitors by a method involving the coating of powders with valve-metal by a chemical vapor reaction process. In the case of coating with tantalum, this is typically effected by reacting a tantalum halide with hydrogen. In the case of coating with aluminum, this can be effected by decomposing aluminum tri-isobutyl into aluminum and isobutylene. Typically the powder to be coated is supported in a fluidized bed.
A significant problem with the aluminum deposition process is that the starting compound is organic, and therefore the product is very susceptible to contamination with carbon. In the case of tantalum deposition, one of the problems is that of contamination arising from the highly reactive halide vapors reacting with materials from which the coating apparatus is made.